This invention relates to sensors for use in plasma spray jet processes and electric arc melting processes. More particularly, the present invention relates to infrared (IR) sensors and plasma spray jet torches controlled with the assistance of such sensors so as to ensure that particles arriving at the substrate to be coated are in a molten state. The invention also relates to infrared sensors and electric arcs (including plasma melting arcs or plumes) controlled with the assistance of such sensors fo measure the temperature and size of molten metal pools and to the control of foreign matter thereon to ensure the pouring of substantially foreign matter-free streams of the molten metal as into ingots. Other variables may be also monitored and controlled.
In plasma spray processes for coating various substrate materials with protective coatings, particulate matter is injected into a plasma-jet which is directed at the substrate. The substrate and particulate matter are typically metal with the particulate matter forming a complex protective coating on the substrate. The protective coating often exhibits superior properties of wear resistance or resistance to corrosion, for example. The structure and properties of the particulate coating are complex functions of plasma torch operating conditions such as the nature and the flow rate of the gases employed, the powder particle size and particle size distribution, the torch-to-substrate distance, electrical power supplied to the torch, and powder injection position, velocity and direction. Because of the large number of interrelated variables, the problem of controlling such a process is exceedingly difficult but for one observation: namely, that the particles must arrive at the substrate in a molten condition. Solid particles reaching the substrate can form, in effect, a defect n the coating. The minimum particle temperature in the plasma is perhaps the singlemost important parameter determining the integrity of the coating but until now, there was no effective means to monitor this parameter. This is due to the overwhelming infrared radiation levels emitted by the plasma as opposed to the particulate matter entrained in the plasma flow.
In short, the plasma spray process has been developed from flame spraying processes largely by empirical methods. The control of such processes is often accomplished through the sole use of empirical methods to ensure that the particles arrive at the substrate in a molten condition. However, because of the large number of variables occurring in such processes, automatic control has hitherto not been employed to control plasma spray jet processes.
In melting technology using heat sources such as electric and plasma arcs, there is a need to measure and to control the size and temperature of molten pools of metals and metallic alloys prior to pouring from their crucibles or hearths to form ingots. In order to produce metallic ingots having improved cleanliness, particularly reduced levels of oxide, there is an increasing need to restrict foreign matter floating on the surface of the molten pool from entering the molten metal stream poured from the melt crucible. Control of such parameters by prior art sensors is not possible due to the overwhelming infrared radiation levels emitted by the arc compared to that emitted from the molten metal surface and the floating foreign matter. However, the sensor described herein overcomes this problem.
Remote temperature measurements of particle temperatures in a plasma-jet have been attempted in research investigations by A. Vardelle and co-workers at the University of Limoges, France, using near-infrared photomultiplier-based optical pyrometers to deduce particle temperature distributions. However, visible and near-infrared measurements are affected by the radiation emitted by the plasma itself which is several thousand degrees higher (in any conventional temperature units) than the particle temperature. Other experimenters have employed videcon-type cameras for investigations of plasma arcs in arc torch welding. However, prior experimenters in the plasma spray jet processses have not appreciated the fact that the plasma of ionized noble gases emits little or no radiation in the far-infrared wavelengths. Only the continuous Planckian radiation from the entrained particles, that is the coating material, falls within this far-infrared range.
Other work in a related field has been reported in a progress report titled "Improvement of Reliability of Welding by In-Process Sensing and Control (Development of Smart Welding Machines for Girth Welding of Pipes)" submitted to the Department of Energy in June, 1981 by Jose Convert et al. This report describes initial experiments conducted using contact sensors (thermocouples) to probe the temperature distribution near weld puddles and seams. Attempts to use near-infrared photodiodes, described therein, for remote temperature sensing were not successful due to significant optical interference from plasma radiation reflected from the metal surface. In particular, Convert et al. propose using a simple optical filter to reduce the radiation from the plasma arc through use of materials similar to conventional welders' goggles.